Hopefully the last in the sordid history of maas tag support. Revno 616 reverted
attempts to string serialize tags for urls, but which had broken constraint
comparisions. Those string serialization had been attempt in revno 597 and 598
to encode maas tags properly for maas client usage. This branch properly
captures the raw tags string for use by the client, while preserving the
constraint comparisons restored in revno 616.

Previously when unit agents were being notified of multiple relations, typicaly
at startup, they would process them in essentially random order. This would
complicate things for applications that would have peer relations as well, for
ha or replication, as they couldn't tell if they were being run standalone or
in a cluster/quorum. Instead when a unit has multiple relations, we process
peer relations first followed by client/server relations.